1. Field of the Invention
The present invention relates to a pulse width modulation circuit for, for example, performing a pulse width modulation (PWM) on an audio signal to output the modulated signal, and a switching amplifier using the same (e.g., an audio amplifier).
2. Description of the Related Art
For example, a type of a conventional switching amplifier proposed in the art uses a pulse width modulation circuit for performing a pulse width modulation on an audio signal as the input signal to output the modulated signal (see, for example, Japanese Laid-Open Patent Publication No. 2004-320097). With the switching amplifier, a predetermined power source voltage is switched ON and OFF based on the modulated signal output from the pulse width modulation circuit, and the output signal switched ON and OFF is output to a load (e.g., a speaker) through a low-pass filter, for example.
FIG. 18 shows a configuration of an example of a conventional switching amplifier. The switching amplifier includes a pulse width modulation circuit 51 connected to an audio signal generation source AU, a switching circuit 52, and a low-pass filter 53. With the switching amplifier, an audio signal eS output from the audio signal generation source AU is input to the pulse width modulation circuit 51, where a pulse width modulation is performed on the amplitude of the audio signal eS. As a result, a modulated signal OUT1 and a modulated signal OUT2 in opposite phase to the modulated signal OUT1 are output to the switching circuit 52.
In the switching circuit 52, the positive and negative power source voltages +VD and −VD are alternately switched ON and OFF by switches SW-a and SW-b, respectively, based on the modulated signals OUT1 and OUT2. The output being switched ON and OFF passes through the low-pass filter 53, where a high-frequency component thereof is removed, and is supplied to a load (not shown) as an output signal
FIG. 19 is a circuit diagram showing a schematic configuration of the pulse width modulation circuit 51 shown in FIG. 18.
The pulse width modulation circuit 51 is an integration-type pulse width modulation circuit using an a stable multivibrator, for example, for performing a pulse width modulation on the audio signal eS as the input signal to produce and output the modulated signal OUT1, for example.
As shown in FIG. 19, the pulse width modulation circuit 51 includes a bias current source 54, a modulation circuit 55 connected to the bias current source 54, and a pulse generation circuit 56 connected to the modulation circuit 55. The bias current source 54 is for supplying a bias current to the modulation circuit 55.
The modulation circuit 55 is implemented by a so-called differential amplifier circuit, and includes resistors R51 and R52 first ends of which are connected to each other, and transistors Q51 and Q52 connected to the second ends of the resistors R51 and R52, respectively. The modulation circuit 55 varies the current distribution ratio between first and second currents I1 and I2, which flow through the transistors Q51 and Q52, respectively, according to the audio signal eS.
The pulse generation circuit 56 is a circuit for producing a pulse signal being a signal to be modulated, i.e., a carrier of a PWM signal, and includes first and second charge condensers C51 and C52, first and second inverters INV51 and INV52, first and second diodes D51 and D52, and a power source voltage 57. The pulse generation circuit 56 charges the first and second charge condensers C51 and C52 based on the first and second currents I1 and I2 supplied from the modulation circuit 55 to output the modulated signal OUT1 whose pulse width corresponds to the charging time of the first charge condenser C51. Note that the predetermined power source voltage 57 is connected to the cathode side of each of the first and second diodes D51 and D52.
With the conventional pulse width modulation circuit 51, the frequency f of the carrier of a PWM signal (hereinafter referred to as the “carrier frequency”) is dependent on the bias current of the bias current source 54, the capacitances of the first and second condensers C51 and C52, the threshold voltages Vth of the first and second inverters INV51 and INV52, etc. Therefore, where the pulse width modulation circuit 51 is used in a multi-channel switching amplifier having a plurality of channels, the carrier frequency f will vary slightly among different channels if there are variations in the capacitances of the first and second condensers C51 and C52, etc.
FIG. 20A shows a spectrum waveform of a carrier in the conventional pulse width modulation circuit 51 where the audio signal eS is absent, and FIG. 20B shows the same during a modulation operation. Note that the expression “the audio signal is absent” as used herein may either mean there is no audio signal being input or refer to a silent portion of an audio signal being input. As shown in the figures, the carrier frequency f is kept at a predetermined frequency f0 when the audio signal eS is absent, but the carrier frequency f is off the predetermined frequency f0 during a modulation operation.
As described above, with the conventional pulse width modulation circuit 51, circuits for different channels separately generate the carrier frequency f. Therefore, the carrier frequency f will vary among different channels. Where the carrier frequency f varies slightly among different channels, a beat component between signals to be modulated (carriers) is mixed in the audio frequency component, and a beat sound may be output as noise. Therefore, a sound whose sound quality is slightly altered is output from the load (speaker) as noise within the audible range.
Moreover, in order to perform an appropriate pulse width modulation operation with the pulse width modulation circuit 51 shown in FIG. 19, charges that have been stored in the first and second charge condensers C51 and C52 during a charge period need to be rapidly discharged to bring the charges in the first and second charge condensers C51 and C52 to a predetermined amount before starting to charge the first and second charge condensers C51 and C52 again.
FIGS. 21A and 21B show voltage waveforms at the point a and the point b in FIG. 19, respectively. The voltage waveform at the point a is a voltage waveform before the second inverter INV52, and the charge stored during a charge period is supposedly discharged and brought back to a predetermined amount. Thus, the voltage before the second inverter INV52 is brought back to the level SL as shown in A1 of FIG. 21A. However, the circuit includes a predetermined time constant, and the charge may not be brought back to a predetermined amount due to the time constant, whereby the next charge period may start before with the stored charge having been not sufficiently discharged (see A2 of FIG. 21A).
Then, the modulated signal OUT1 being a voltage after the second inverter INV52 (see the point b in FIG. 19) will have a pulse width with a conversion error (see B2 of FIG. 21B) as opposed to the normal pulse width (see B1 of FIG. 21B). If such a conversion error occurs during the conversion of the audio signal eS into a PWM signal, it will be impossible to perform an appropriate pulse width modulation, thus significantly influencing the sound quality.
The present invention has been made in view of the above, and it is an object of the present invention to provide a pulse modulation circuit with which the carrier frequency is made substantially constant, thus allowing for an appropriate pulse width modulation, and a switching amplifier using the same.